1. Field of the Invention
The present invention relates to velocity control of a rotary motor contained in a video cassette recorder (VCR), and more particularly, to a velocity control method for a rotary motor and an apparatus adopting the same, in which a disturbance applied to the rotary motor is estimated and corrected to improve a velocity control characteristic.
2. Description of the Related Art
Since disturbances exist in controlling motors, accurate motor control is difficult to achieve in practice and a control error is generated. Therefore, a VCR (Video Cassette Recorder) driven by a motor cannot perform accurately. Thus, disturbance is removed by estimating a torque of the disturbance of the motor by means of an observer. A control theory of suppressing such a load disturbance is being studied. One example is shown in FIG. 1.
FIG. 1 is a block diagram showing a conventional velocity control apparatus of a rotary motor having a disturbance. The apparatus of FIG. 1 is disclosed in a paper entitled, "Autocompensation of Torque Ripple of Direct Drive Motor by Torque Observer" by Nobuyuki Matsui, Tatsuo Makino, and Hirokazu Satoh (IEEE Trans. on Industry Applications, Vol. 29, No. 1, January/February 1993, pp. 187-194). In FIG. 1, a first adder A1 receives a velocity command .omega.*.sub.m as a reference input and an angular velocity .omega..sub.m fed back from a motor 15 and obtains a difference .omega.*.sub.m -.omega..sub.m. The difference .omega.*.sub.m -.omega..sub.m is input to a velocity controller 11. The velocity controller 11 outputs a current command i.sub.v * to control a rotational velocity of the motor 15 according to the input difference .omega.*.sub.m -.omega..sub.m. The current command i.sub.v * is input to a second adder A2. The second adder A2 adds the current command i.sub.v * applied from the velocity controller 11 and a disturbance removal command i.sub.L * and obtains a corrected current command i*. The disturbance removal command i.sub.L * is obtained by multiplying an estimation of load disturbance torque .tau..sub.L (i) estimated in a torque observer 17 which receives an angular velocity .omega..sub.m fed back from the motor 15 and an actual current i by a transfer function K.sub.T .sup.-1. Here, values without the superscript * are actual values and values with the superscript * are command values.
Meanwhile, the corrected current command i* is input to a current controller 13. The current controller 13 expressed as the transfer function K.sub.T supplies a torque command .tau.* to the motor 15 in order to control a rotational velocity of the motor 15 in response to the corrected current command i*. The motor 15 rotates at a speed corresponding to the torque command .tau.* of the current controller 13.
However, in controlling a motor, factors varying a control quantity are generated according to peripheral conditions. Such factors are called disturbances T.sub.L which render accurate velocity control difficult to obtain. Thus, a good velocity characteristic can be obtained only when an effect of the disturbance is removed. The torque observer 17 which generates an estimate of the actual disturbance T.sub.L applied to the motor 15 obtains an estimated load disturbance torque .tau..sub.L (i) according to the following relationships (1) under the assumption that the disturbance is varied sufficiently slowly. ##EQU1##
Jn, Dn and K.sub.Tn are nominal values with respect to an inertial moment J, a damping factor D and a torque constant K.sub.T. L, which is smaller than zero is an observer gain, .epsilon. is a random variable, .tau..sub.L is an observer output, and T.sub.s is a sampling period.
The above relationships (1) can be combined to yield the following relationship (2). ##EQU2##
Here, .tau..sub.L is an actual load disturbance torque, .tau..sub.L is an estimated load disturbance torque and s is a Laplacian operator. In this case, T=-T.sub.s /ln(1+LT.sub.s /J.sub.n) and 1/(1+ST) serves as a low-pass filter. Values without a subscript n are actual values and values with the subscript n are nominal values which are designed to be close to the actual values.
Thus, if the actual load disturbance torque .tau..sub.L is slowly varied, it approximates the estimated load disturbance torque .tau..sub.L to completely remove the load disturbance T.sub.L.
As described above, the conventional method of removing the load disturbance by estimating the load disturbance torque of the motor requires a large calculation time due to the complex equations and thus, there are substantial problems associated with implementing it into hardware. Since a bandwidth of the low-pass filter 1/(1+ST) becomes large to follow up the fast varying disturbances, the observer gain L must be large accordingly. The gain cannot be enlarged without generating limitations in an actual implementation, and the disturbance is estimated with respect to time. As a result, a continuous estimation operation should be performed during the operation of a closed loop with respect to the velocity control of the rotary motor. This requires a great deal of calculation and thus is difficult to implement known hardware.